Csdrhrt

I currently have a Seiki (SE32HS01; provided by my Advanced display options) TV (monitor) hooked up to one of my NVIDIA Geforce GTX 1060 6GB HDMI output. I use it mainly to watch videos if I'm laying in bed so I don't really have it on as much as I do my main monitor (Acer XF270H B; also provided by display options; if you need more detail I can look at the manuals).

I notice though that when it's off and I'm using my computer that it it still holds some of my pages open and I can drag stuff in and out of it. It's as if it was on, but it's off.

Now I would normally just attribute that to the lack of technology on the TV but whenever I do turn the TV on, my computer reinitialized the connection as if it was disconnected and reconnected.

This means that there has to be a way that my computer can know that the TV is off right?

It might require a third party extension, but I would like for my computer to recognize the off screen and return back to a single screen computer when the TV is off and return to the double screen when it is on.

Does anyone know of a windows setting or third party extension I can use to do this?

If your TV is detected by Windows then it is not totally off.
Most modern TV do not totally turn off but leave some circuits working.

To really turn it off you will need to unplug it at the mains.
And if Windows then still claims to detect it, then there is a problem with Windows.

To move all open windows to the main monitor while the TV is unavailable,
you could right-click the taskbar and choose Cascade.
This will arrange all open windows in a cascade, which might be bothersome,
but it will also move them all to the main screen in the process.

When your TV is off it simply puts its electronics into a low power "standby" mode. It doesn't actually properly turn them off.

What that means is that your computer cannot actually detect you turning your TV off.

The reason it detects when your TV turns on is because as part of coming out of standby your TV appears to reset the electronics on the input from your computer. This will cause your computer to see the connection go down briefly and come back up.

If you manually want to revert to a single display you can press Windows+P and select "PC Screen Only" to switch back to single screen mode. When you want to use the TV again you press the key combination again and select "Extend".

There is nothing else you can do without support from your TV or properly pulling the power out of it. Perhaps a wifi controlled power switch?

The video card is responsible for generating,maintaining, and notifying windows how many displays are attached.

NVIDIA has a display detect function built-in to its windows control panel. I suggest turning the TV off, and then clicking detect to see if the second screen is de-activated.

VGA and DVI-D have no way of passing the ON or OFF signal back to the video card.

HDMI has CEC built into it which allows not just the ON/OFF command, but a variety of commands.

Displayport has a similar option but I forgot what its called.

If video cards exposed the option, you could use HDMI and CEC to turn the monitor off from the computer.

For example, I can use my TV remote to turn ON my blu-ray player, and control it via the TV's remote because of the CEC function. In addition, if I place a DVD/BLURAY disc in, and the TV is OFF, it automatically turns ON with the contents of the disc displayed.
From some unknown reason each manufacturer has a different name for CEC in the marketing materials.

When a display is plugged in it exchanges EDID formatted information to let the other device know what it can handle in terms of resolutions and frequencies and a small amount of trivial data. EDID has been supported since VGA.

As an alternative, it may be useful to you to switch between Mirrored and Extended modes using Windows key+P when the TV is turned off and on. As mentioned in another answer, the TV may appear to be "on" to the video card when the screen itself is off.